[SOLVED] Can send external but not send/recieve internal email

Hi, I'm new to this and I have been giving a task on setting up email on Zimbra using Fedora 7. I got Fedora and Zimbra installed correctly and have successfully sent out emails to an external email address.

When I try to reply back I get "undelivered mail returned to sender." When I try to create a new email from an external accountand send it off from my test account I get "The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator."

I also have not been getting any response when attempting to send emails internally. I'm gonna need some major help and Thanks to everyone in advanced.

If you listed your domain we could check mx records for you - or you can do a "dig mx domain.com"

Can you telnet to port 25 on that box? (from the outside - not any boxes on your lan)
If not: firewall or even your hosting provider blocking port 25?

Do a tail -f /var/log/zimbra.log do you see the connection attempts? what are the errors there?
-check the queue's
If that appears ok, then tail -f /opt/zimbra/logs/mailbox.log do you see the mail showing up there?

Domain is sleeplms.net
I can telnet to port 25 from outside the network.
When I do a tail to the zimbra.log nothing is there
When I do a tail to the mailbox.log I do not see any error messages.
The box does have a public and private address. I will read about splitting DNS. Is there anything else I should look at?

MMorse, I would like to thank you for your help. That web page pingability informed me that I had some MX records problems. So I reconfig my dns and its finally working. I will do more testing but it looks pretty stable now.